[Xen-users] Re: Snapshotting LVM backed guests from dom0

["Nick Couchman" <Nick.Couchman@xxxxxxxxx>]

> On Sat, Apr 17, 2010 at 2:53 PM, chris <tknchris@xxxxxxxxx> wrote:
>> Just looking for some feedback from other people who do this. I know
>> its not a good "backup" method but "crash consistent" images have been
>> very useful for me in disaster situations just to get OS running
>> quickly then restore data from a data backup. My typical setup is to
>> put the LV in snapshot mode while guest is running then dd the data to
>> a backup file which is on a NFS mount point. The thing that seems to
>> be happening is that the VM's performance gets pretty poor during the
>> time the copy is happening. My guesses at why this was happening were:
>>
>> 1.   dom0 having equal weight to the other 4 guests on the box and
>> somehow hogging cpu time
>> 2.   lack of QoS on the IO side / dom0 hogging IO
>> 3.   process priorities in dom0
>> 4.   NFS overhead
>>
>> For each of these items I tried to adjust things to see if it improved.
>>
>> 1.   Tried increasing dom0 weight to 4x the other VM's.

Probably not going to help - if you increase the weight, you'll choke out your 
other domUs, if you decrease the weight, the domUs also may be affected because 
network and disk I/O end up going through dom0 in the end, anyway.

>> 2.   Saw pasi mentioning dm-ioband a few times and think this might
>> address IO scheduling but haven't tried it yet.
>> 3.   Tried nice-ing the dd to lowest priority and qemu-dm to highest

I would expect this to help, some, but may not be the only thing.  Also, 
remember that network and disk I/O are still done through drivers on dom0, 
which means pushing qemu-dm to the highest really won't buy you anything.  I 
would expect re-niceing dd to help some, though.

>> 4.   Changing destination to a local

This indicates that the bottleneck is local and not the network.  The next step 
would be to grab some Linux performance monitoring and debugging tools and 
figure out where your bottleneck is.   So, things like top, xentop, iostat, 
vmstat, and sar may be useful in determining what component is hitting its 
performance limit and needs to be tweaked or worked around.

-Nick




--------
This e-mail may contain confidential and privileged material for the sole use 
of the intended recipient.  If this email is not intended for you, or you are 
not responsible for the delivery of this message to the intended recipient, 
please note that this message may contain SEAKR Engineering (SEAKR) 
Privileged/Proprietary Information.  In such a case, you are strictly 
prohibited from downloading, photocopying, distributing or otherwise using this 
message, its contents or attachments in any way.  If you have received this 
message in error, please notify us immediately by replying to this e-mail and 
delete the message from your mailbox.  Information contained in this message 
that does not relate to the business of SEAKR is neither endorsed by nor 
attributable to SEAKR.

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users